This invention relates to a method and system for feeding comminuted cellulosic fibrous material (“chips”) to a treatment vessel, such as a continuous digester.
The reaction of pulping chemicals with comminuted cellulosic fibrous material to produce a chemical pulp requires temperatures ranging between 140-180 degree Celsius (C.). Since the aqueous chemicals used to treat the material boil at such temperatures, commercial chemical pulping is typically performed in a pressure-resistant vessel, e.g., a continuous digester, under pressures of at least about 10 bars gauge (approximately 150 psi gauge). To maintain this pressure, especially when performing a continuous pulping process, a high pressure feeder (HPF) increases the pressure of the chip slurry entering the vessel, e.g., a digester, to a pressure level at or above the pressure in the vessel to ensure that the pressure is not lost when introducing material to the pressure vessel.
The present invention relates to the transfer system for feeding chips to a high pressure continuous digester and/or to other high pressure chip processing systems. High pressure chip processing systems typically include a HPF, such as is shown in U.S. Pat. No. 6,669,410. The HPF receives a low-pressure slurry of comminuted cellulosic fibrous material (“chip slurry”) and outputs a high-pressure chip slurry. The high pressure slurry is suitable for introduction into a continuous digester, chip steaming vessel and other high pressure chip processing systems.
Typically, high pressure feeders contain a pocketed rotor which acts as a means for transferring a slurry of material from a low pressure to a high pressure while also acting as a valve for preventing loss of pressure. The rotor has a chamber for transferring low pressure slurry to a high pressure stream. An HPF generally has a stationary housing with a low pressure inlet port at its top (12:00 position), low pressure outlet port at its bottom (6:00 position), a high pressure inlet port at a first side (9:00 position) and an high pressure outlet port at an opposite side (3:00 position). A rotor in the feeder housing alternately opens the pair of low pressure inlet and outlet ports and then opens the pair of high pressure inlet and outlet ports. The low pressure ports are not open while the high pressure ports are open and vice versa. When the low pressure inlet and outlet ports are opened, a new volume of chip slurry enters the rotor chamber and some liquid is purged through the outlet. When the rotor opens the high pressure inlet and outlet, high pressure liquid enters and flushes the chip slurry in the rotor chamber through the high pressure outlet into a high pressure conduit.
The top port (12:00) of the feeder housing of the HPF is the low-pressure inlet port into which a slurry of chips and liquid is introduced to the feeder. The top port has historically been the low pressure chip slurry input. However, due to the pump-feeding which characterizes the LO-LEVEL™ Feed System marketed by Andritz Inc. of Glens Falls, N.Y., the pressurized slurry flow from the slurry pump may be introduced to a low-pressure inlet of the HPF which is oriented wherever necessitated by the installation. The pump-fed slurry can be introduced to a port located physically on top, on either side, on the bottom of the HPF, or even to a port oriented at an oblique angle, that is, at any angle of orientation desired.
As the low-pressure slurry is introduced to the low-pressure inlet of the HPF, one or more of the through-going pockets of the rotating rotor receive the slurry. The low-pressure outlet of the HPF is located opposite the low-pressure inlet. As the slurry is introduced to the low-pressure inlet and the first end of one of the through-going pockets, the slurry flows into the rotor pocket and toward a second and opposite end of the pocket, in this case, toward the lower end of the pocket, and toward the low-pressure outlet. The low-pressure outlet port of the HPF is typically provided with a screen element, for example, a cast horizontal bar type screen element (see for example the screen element in U.S. Pat. No. 5,443,162). This screen element retains the chips in the slurry within the rotor of the feeder and allows some of the liquid in the slurry to pass out of the second end of the pocket and through the screen. The liquid discharged from the low-pressure outlet has in the past been recirculated back into the chip feed system to a location upstream of the HPF in the flow of the chip slurry. A difficulty with the screen for the low pressure outlet is that some chips pass through the screen. These chips are then unavailable for further processing in the digester.
The chips that are introduced to the rotor pocket of the HPF, including those chips retained by the screen element, are transported by the rotation of the rotor. After a typical one-quarter revolution of the rotor, the first end of the pocket that was once in communication with the low-pressure inlet is in communication with the HPF high pressure outlet. The high-pressure outlet typically communicates with the inlet of a digester, either a continuous or batch digester, via one or more conduits. At the same time, the rotation of the rotor also places the second end of the through-going pocket, which was just in communication with the low-pressure outlet, in communication with the high-pressure inlet. The high pressure inlet typically receives a flow of high-pressure liquid from a high-pressure hydraulic pump. The pressure of this liquid typically ranges from about 5 to 15 bar gauge, and is typically about 7-10 bar gauge. This high-pressure liquid displaces the slurry of chips and liquid from the through-going pocket and out of the high-pressure outlet and ultimately to the inlet of the digester.
As the rotor continues to rotate, the second end of the pocket which received the high-pressure fluid then is placed in communication with the low-pressure inlet and receives another supply of slurry from the conduit connected to the low-pressure inlet. Similarly, the first end of the pocket is rotated into communication with the low-pressure outlet of the housing, having the screen element. The process described above then repeats itself such that during one complete revolution of the rotor each through-going pocket receives and discharges two charges of chips and liquid. The rotor typically contains at least two, typically four, through-going pockets such that the rotor is repeatedly receiving slurry from the low-pressure inlet and discharging slurry out the high-pressure outlet. The ends of the these pockets act as both an inlet for slurry and an outlet depending upon the orientation of the rotor.
A difficulty has arisen in certain HPFs operating at relatively high rotor speeds. The difficulty is that excessive amounts of chips tend to accumulate on and pass through the screen in the HPF housing at the low-pressure outlet (6:00 position). The chips in the low pressure discharge conduit are lost to the chip processing process and can clog the liquor processing equipment receiving the low pressure discharge of the HPF.